Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic projection apparatus is disclosed in which a space between the projection system and a sensor is filled with a liquid.

This application is a continuation application of co-pending U.S. patentapplication no. 13/194,136, filed Jul. 29, 2011, which is a continuationapplication of co-pending U.S. patent application Ser. No. 12/698,932,filed Feb. 2, 2010, which is a continuation application of co-pendingU.S. patent application Ser. No. 11/482,122, filed Jul. 7, 2006, whichis a continuation application of U.S. patent application Ser. No.10/857,614, filed Jun. 1, 2004, now U.S. Pat. No. 7,213,963, which inturn claims priority from European patent applications EP 03253636.9,filed Jun. 9, 2003, EP 03255395.0, filed Aug. 29, 2003, and EP03257068.1, filed Nov. 10, 2003, each foregoing application incorporatedherein in its entirety by reference.

FIELD

The present invention relates to a lithographic projection apparatus anda device manufacturing method.

BACKGROUND

The term “patterning device” as here employed should be broadlyinterpreted as referring to any device that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such a patterning device include:

A mask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

A programmable mirror array. One example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, theundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. An alternative embodiment of a programmable mirror arrayemploys a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuation means. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors; in this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-addressable mirrors.The matrix addressing can be performed using suitable electronics. Inboth of the situations described hereabove, the patterning device cancomprise one or more programmable mirror arrays. More information onmirror arrays as here referred to can be gleaned, for example, from U.S.Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patentapplication publications WO 98/38597 and WO 98/33096, which areincorporated herein by reference. In the case of a programmable mirrorarray, the support structure may be embodied as a frame or table, forexample, which may be fixed or movable as required.

A programmable LCD array. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference. Asabove, the support structure in this case may be embodied as a frame ortable, for example, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at one time; such an apparatus is commonly referredto as a stepper. In an alternative apparatus—commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “projection lens”; however, this term should bebroadly interpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components maybe referred to below, collectively or singularly, as a “lens”. Further,the lithographic apparatus may be of a type having two or more substratetables (and/or two or more mask tables). In such “multiple stage”devices the additional tables may be used in parallel, or preparatorysteps may be carried out on one or more tables while one or more othertables are being used for exposures. Dual stage lithographic apparatusare described, for example, in U.S. Pat. No. 5,969,441 and PCT patentapplication publication WO 98/40791, incorporated herein by reference.

It has been proposed to immerse the substrate in a lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill the space between the final opticalelement of the projection system and the substrate. The point of this isto enable imaging of smaller features because the exposure radiationwill have a shorter wavelength in the liquid than in gas (e.g., air) orin a vacuum. (The effect of the liquid may also be regarded asincreasing the effective NA of the system).

However, submersing the substrate or substrate and substrate table in abath of liquid (see for example U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application publication WO 99/49504, herebyincorporated in its entirety by reference. As illustrated in FIGS. 14and 15, liquid is supplied by at least one inlet IN onto the substrate,preferably along the direction of movement of the substrate relative tothe final element, and is removed by at least one outlet OUT afterhaving passed under the projection system. That is, as the substrate isscanned beneath the element in a −X direction, liquid is supplied at the+X side of the element and taken up at the −X side. FIG. 15 shows thearrangement schematically in which liquid is supplied via inlet IN andis taken up on the other side of the element by outlet OUT which isconnected to a low pressure source. In the illustration of FIG. 14 theliquid is supplied along the direction of movement of the substraterelative to the final element, though this does not need to be the case.Various orientations and numbers of in- and out-lets positioned aroundthe final element are possible, one example is illustrated in FIG. 23 inwhich four sets of an inlet with an outlet on either side are providedin a regular pattern around the final element.

SUMMARY

Accordingly, it would be advantageous, for example, to provide animmersion lithographic projection apparatus with improved functionality.

According to an aspect of the invention, there is provided alithographic projection apparatus comprising:

an illuminator adapted to condition a beam of radiation;

a support structure configured to hold a patterning device, thepatterning device configured to pattern the beam of radiation accordingto a desired pattern;

a substrate table configured to hold a substrate;

a projection system adapted to project the patterned beam onto a targetportion of the substrate;

a liquid supply system configured to at least partly fill a spacebetween the projection system and an object on the substrate table, witha liquid; and

a sensor capable of being positioned to be illuminated by the beam ofradiation once it has passed through the liquid.

By passing a beam of radiation for a sensor through liquid, no elaboratemeasures need to be taken to compensate the signals from the sensor totake account of the parameters measured by the sensor being measuredthrough a different medium to that which the substrate is imagedthrough. However, it may be necessary to ensure that the design of thesensor is such that it is compatible for use when covered with liquid.An example sensor includes an alignment sensor configured to align thesubstrate table relative to the projection system, a transmission imagesensor, a focus sensor, a spot or dose sensor, an integrated lensinterferometer and scanner sensor and even an alignment mark. In thecase of an alignment sensor, the measurement gratings of the sensor mayhave a pitch than less than 500 nm, such pitch possibly improving theresolution of the alignment sensor.

According to a further aspect of the present invention, there isprovided a device manufacturing method comprising:

projecting a beam of radiation through a liquid onto a sensor; and

projecting the beam of radiation as patterned using a projection systemof a lithographic apparatus through the liquid onto a target portion ofa substrate.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts the liquid reservoir of a first embodiment of theinvention;

FIG. 3 illustrates a second embodiment of the invention;

FIG. 4 illustrates an alternative form of the second embodiment of thepresent invention;

FIG. 5 illustrates a detail of the second embodiment of the presentinvention;

FIG. 6 a illustrates a first version of a third embodiment of thepresent invention;

FIG. 6 b illustrates a second version of the third embodiment;

FIG. 6 c illustrates a third version of the third embodiment;

FIG. 7 illustrates in detail further aspects of the first version of thethird embodiment of the present invention;

FIG. 8 illustrates a fourth embodiment of the present invention;

FIG. 9 illustrates an fifth embodiment of the present invention;

FIG. 10 illustrates a sixth embodiment of the present invention;

FIG. 11 illustrates a seventh embodiment of the present invention;

FIG. 12 illustrates an eighth embodiment of the present invention;

FIG. 13 illustrates a eighth embodiment of the present invention;

FIG. 14 illustrates an alternative liquid supply system according to anembodiment of the invention;

FIG. 15 illustrates, in plan, the system of FIG. 14;

FIG. 16 depicts an ILIAS sensor module;

FIG. 17 depicts an ILIAS sensor module with an elongated transmissiveplate according to an embodiment of the present invention;

FIG. 18 depicts an ILIAS sensor module with a filler sheet according toan embodiment of the present invention; and

FIGS. 19 a and 19 b depict a luminescence based DUV transmission imagesensor.

DETAILED DESCRIPTION Embodiment 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

a radiation system Ex, IL, for supplying a projection beam PB ofradiation (e.g. DUV radiation), which in this particular case alsocomprises a radiation source LA;

a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g. a reticle), and connected to a first positioningdevice for accurately positioning the mask with respect to item PL;

a second object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g. a resist-coated silicon wafer),and connected to a second positioning device for accurately positioningthe substrate with respect to item PL;

a projection system (“projection lens”) PL (e.g. a refractive system)for imaging an irradiated portion of the mask MA onto a target portion C(e.g. comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning device, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. an excimer laser) produces a beam of radiation. Thisbeam is fed into an illumination system (illuminator) IL, eitherdirectly or after having traversed conditioning means, such as a beamexpander Ex, for example. The illuminator IL may comprise adjustingmeans AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam. In addition, it will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass at least both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through theprojection system PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the second positioning device (andinterferometric measuring device IF), the substrate table WT can bemoved accurately, e.g. so as to position different target portions C inthe path of the beam PB. Similarly, the first positioning device can beused to accurately position the mask MA with respect to the path of thebeam PB, e.g. after mechanical retrieval of the mask MA from a masklibrary, or during a scan. In general, movement of the object tables MT,WT will be realized with the aid of a long-stroke module (coursepositioning) and a short-stroke module (fine positioning), which are notexplicitly depicted in FIG. 1. However, in the case of a stepper (asopposed to a step-and-scan apparatus) the mask table MT may just beconnected to a short stroke actuator, or may be fixed.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at one time (i.e. a single “flash”)onto a target portion C. The substrate table WT is then shifted in the Xand/or Y directions so that a different target portion C can beirradiated by the beam PB;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the Y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the projection system PL(typically, M=¼ or ⅕). In this manner, a relatively large target portionC can be exposed, without having to compromise on resolution.

FIG. 2 shows a liquid reservoir 10 between the projection system PL andthe substrate W which is positioned on the substrate stage WT. Theliquid reservoir 10 is filled with a liquid 11 having a relatively highrefractive index, e.g. water or a suspension of particles in water,provided via inlet/outlet ducts 13. The liquid has the effect that theradiation of the projection beam is a shorter wavelength in the liquidthan in gas (e.g., air) or in a vacuum, allowing smaller features to beresolved. It is well known that the resolution limit of a projectionsystem is determined, inter alia, by the wavelength of the projectionbeam and the numerical aperture of the system. The presence of theliquid may also be regarded as increasing the effective numericalaperture. Furthermore, at fixed numerical aperture, the liquid iseffective to increase the depth of field.

The reservoir 10 forms, in an embodiment, a contactless seal to thesubstrate W around the image field of the projection system PL so thatthe liquid is confined to fill the space between the substrate's primarysurface, which faces the projection system PL, and the final opticalelement of the projection system PL. The reservoir is formed by a sealmember 12 positioned below and surrounding the final element of theprojection system PL. Thus, the liquid supply system provides liquid ononly a localized area of the substrate. The seal member 12 forms part ofthe liquid supply system for filling the space between the final elementof the projection system and the substrate with a liquid. This liquid isbrought into the space below the projection system and within the sealmember 12. In an embodiment, the seal member 12 extends a little abovethe bottom element of the projection system and the liquid rises abovethe final element so that a buffer of liquid is provided. The sealmember 12 has an inner periphery that at the upper end closely conformsto the shape of the projection system or the final elements thereof andmay, e.g. be round. At the bottom the inner periphery closely conformsto the shape of the image field, e.g. rectangular, though this is notnecessarily so. The seal member is substantially stationary in the XYplane relative to the projection system though there may be somerelative movement in the Z direction (in the direction of the opticalaxis). A seal is formed between the seal member and the surface of thesubstrate. In an implementation, this seal is a contactless seal and maybe a gas seal.

The liquid 11 is confined in the reservoir 10 by a seal device 16. Asillustrated in FIG. 2, the seal device is a contactless seal i.e. a gasseal. The gas seal is formed by gas, e.g. air or synthetic air, providedunder pressure via inlet 15 to the gap between seal member 12 andsubstrate W and extracted by first outlet 14. The over pressure on thegas inlet 15, vacuum level or under pressure on the first outlet 14 andthe geometry of the gap are arranged so that there is a high-velocitygas flow inwards towards the optical axis of the apparatus that confinesthe liquid 11. As with any seal, some liquid is likely to escape, forexample up the first outlet 14.

FIGS. 14 and 15 also depict a liquid reservoir defined by inlet(s) IN,outlet(s) OUT, the substrate W and the final element of projectionsystem PL. Like the liquid supply system of FIG. 2 the liquid supplysystem illustrated in FIGS. 14 and 15, comprising inlet(s) IN andoutlet(s) OUT, supplies liquid to the primary surface of the substratein a localized area between the final element of the projection systemand the substrate and can suffer from loss of liquid at the substrateedge.

Thus, as used herein for the embodiments, the liquid supply system cancomprise that as described in relation to FIG. 2 and FIGS. 14 and 15.

Embodiment 2

A second embodiment is illustrated in FIGS. 3 to 5 and is the same orsimilar as the first embodiment except as described below.

In the embodiment of FIGS. 3 and 4 an edge liquid supply system providesliquid to a reservoir 30 via a port 40. The liquid in the reservoir 30is optionally the same as the immersion liquid in the liquid supplysystem. The reservoir 30 is positioned on the opposite side of thesubstrate W to the projection system PL and adjacent the edge of thesubstrate W and the edge of the edge seal member 17, 117. In FIG. 4, theedge seal member 17 is comprised of an element which is separate to thesubstrate table WT whereas in FIG. 3 the edge seal member 117 isprovided by an integral portion of the substrate table WT. As can beseen most clearly from FIG. 3, the substrate W is supported on thesubstrate table WT by a so-called pimple table 20. The pimple table 20comprises a plurality of projections on which the substrate W rests. Thesubstrate W is held in place by, e.g., a vacuum source sucking thesubstrate onto the top surface of the substrate table WT. With the useof the reservoir 30, when the edge of the substrate W is being imaged,(i.e. when liquid in the liquid supply system between the projectionsystem PL and the substrate W traverses across an edge of the substrateW), liquid cannot escape from the liquid supply system into the gapbetween the edge seal member 17, 117 and the substrate W because thatspace is already filled with liquid.

The mechanism 170 shown in FIG. 4 for moving the edge seal member 17relative to the remainder of the substrate table WT is illustrated indetail in FIG. 5. The reason for moving the edge seal member 17 in thisway is so that its primary surface can be made to be substantiallyco-planar with the primary surface of the substrate W. This allows asmooth movement of the liquid supply system over edge portions of thesubstrate W so that the bottom inner periphery of the liquid supplysystem can be moved to positions partly on the primary surface ofsubstrate W and partly on the primary surface of the edge seal member17.

A level sensor (not illustrated) is used to detect the relative heightsof the primary surfaces of the substrate W and the edge seal member 17.Based on the results of the level sensor, control signals are sent tothe actuator 171 in order to adjust the height of the primary surface ofthe edge seal member 17. A closed loop actuator could also be used forthis purpose.

In an implementation, the actuator 171 is a rotating motor which rotatesa shaft 176. The shaft 176 is connected to a circular disc at the enddistal to the motor 171. The shaft 176 is connected away from the centreof the disc. The disc is located in a circular recess in a wedge portion172. Ball bearings may be used to reduce the amount of friction betweenthe circular disc and the sides of the recess in the wedge portion 172.The motor 171 is held in place by leaf springs 177. On actuation of themotor the wedge portion is driven to the left and right as illustrated(i.e. in the direction of the slope of the wedge portion) because of theexcentre position of the shaft 176 in the disc. The motor is preventedfrom moving in the same direction as the direction of movement of thewedge portion 172 by the springs 177.

As the wedge portion 172 moves left and right as illustrated in FIG. 5,its top surface 175 (which is the surface of the wedge which is slopedin relation to the primary surface of the edge seal member 17) contactsthe bottom sloped surface of a further wedge member 173 which is fixedto the bottom of the edge seal member 17. The edge seal member 17 isprevented from moving in the direction of movement of the wedge member172 so that when the wedge member 172 moves left and right the edge sealmember 17 is lowered and raised respectively. Some biasing of the edgeseal member 17 towards the substrate table WT may be necessary.

Obviously the further wedge member 173 could be replaced by analternative shape, for example a rod positioned perpendicularly to thedirection of movement of the wedge 172. If the coefficient of frictionbetween the wedge member 172 and the further wedge member 173 is greaterthan the tangent of the wedge angle then the actuator 170 isself-braking meaning that no force may be needed on the wedge member 172to hold it in place. This is advantageous as the system will then bestable when the actuator 171 is not actuated. The accuracy of themechanism 170 is of the order of a few μm.

Especially in the case of the edge seal member 117 being an integralpart of the substrate table WT, a mechanism may be provided to adjustthe height of the substrate W or the member supporting the substrate Wso that the primary surfaces of the edge seal member 17, 117 and thesubstrate can be made substantially co-planar.

Embodiment 3

A third embodiment is illustrated in FIGS. 6 and 7 and is the same orsimilar as the first embodiment except as described below.

This embodiment is described in relation to an edge seal member 117which is an integral part of the substrate table WT. However, thisembodiment is equally applicable to an edge seal member 17 which ismovable relative to the substrate table WT.

In a first version of this embodiment as illustrated in FIG. 6 a, afurther edge seal member 500 is used to bridge the gap between the edgeseal member 117 and the substrate W. The further edge seal member isaffixed to the edge seal member 117. The further edge seal member 500 isremovably attachable against the surface of the substrate W opposite theprimary surface. In this embodiment the further edge seal member 500 canbe a flexible edge seal member which is actuatable to contact the undersurface of the substrate W. When the flexible edge seal member 500 isdeactivated it falls away from the substrate under gravity. The way thisis achieved is illustrated in FIG. 7 and is described below.

It is likely that the further edge seal member 500 will not prevent allof the immersion liquid from the liquid supply system from entering thespace under the substrate W and for this reason a port 46 connected to alow pressure source may be provided under the substrate W adjacent edgesof the edge seal member 117 and the substrate W in some or all of theversions of this embodiment. Of course the design of the area under thesubstrate could be the same as that of the second embodiment.

The same system can be used for sensors such as a transmission imagesensor (TIS) on the substrate table as opposed for the substrate W. Inthe case of sensors, as the sensors do not move, the further edge sealmember 500 can be permanently attached to the sensor, for example usingglue.

Furthermore, the further edge seal member 500 can be arranged to engagewith the top surface of the object (that surface closest to theprojection system) rather than the bottom surface. Also, the furtheredge seal member 500 may be provided attached to or near the top surfaceof the edge seal member 117 as opposed to under the edge seal member 117as is illustrated in FIG. 6 a.

A second version of this embodiment is illustrated in FIG. 6 b. Twofurther edge seal members 500 a, 500 b are used. The first of these edgeseal members 500 a is the same as in the first version. The second ofthese edge seal members 500 b is affixed to the substrate table 20 i.e.underneath the substrate W and extends with its free end radiallyoutwardly from its attachment point. The second further edge seal member500 b clamps the first further edge seal member 500 a against thesubstrate W. Compressed gas can be used to deform or move the secondfurther edge seal member 500 b.

A third version of this embodiment is shown in FIG. 6 c. The thirdversion is the same as the second version except the first further edgeseal member 500 c clamps the second further edge seal member 500 d tothe substrate W. This avoids, for example, the need for the compressedgas of the second version.

It will be appreciated that the embodiment will also work with only thesecond further edge seal member 500 b, 500 d with or without connectionto vacuum.

Various ways of deforming the further edge seal members 500, 500 a, 500b, 500 c, 500 d will now be described in relation to the first versionof the embodiment.

As can be seen from FIG. 7, a channel 510 is formed in the elongatedirection of a flexible further edge seal member 500 (which, in animplementation, is an annular ring) and (a) discrete port(s) areprovided in an upper surface of the flexible further edge seal memberwhich faces the projection system PL and the underside of the substrateW. By connecting a vacuum source 515 to the duct 510 the flexiblefurther edge seal member can be made to abut the substrate W by suction.When the vacuum source 515 is disconnected or switched off, the flexiblefurther edge seal member 500 drops under gravity and/or pressure fromport 46 to assume the position shown in dotted lines in FIG. 7.

In an alternative or additional embodiment, a flexible further edge sealmember 500 is formed with a mechanical pre-load such that it contactsthe substrate W when the substrate is placed on the pimple table 20 andthe flexible further edge seal member 500 deforms elastically so that itapplies a force upwards on the substrate W to thereby make a seal.

In a further alternative or additional embodiment, a flexible furtheredge seal member 500 may be forced against the substrate W by anoverpressure generated by pressurised gas on port 46.

A flexible further edge seal member 500 may be fashioned from anyflexible, radiation and immersion liquid resistant, non-contaminatingmaterial, for example, steel, glass e.g. Al₂O₃, ceramic material e.g.SiC, Silicon, Teflon, low expansion glasses (e.g. Zerodur™ or ULE™),carbon fibre epoxy or quartz and is typically between 10 and 500 μmthick, optionally between 30 and 200 μm or 50 to 150 μm in the case ofglass. With a flexible further edge seal member 500 of this material andthese dimensions, the typical pressure to be applied to the duct 510 isapproximately 0.1 to 0.6 bar.

Embodiment 4

A fourth embodiment is illustrated in FIG. 8 and is the same or similaras the first embodiment except as described below.

This embodiment is described in relation to an edge seal member 117which is an integral part of the substrate table WT. However, thisembodiment is equally applicable to an edge seal member 17 which ismovable relative to the substrate table WT.

In the fourth embodiment, the gap between the edge seal member 117 andthe substrate W is filled with a further edge seal member 50. Thefurther edge seal member is a flexible further edge seal member 50 whichhas a top surface which is substantially co-planar with the primarysurfaces of the substrate W and the edge seal member 117. The flexiblefurther edge seal member 50 is made of a compliant material so thatminor variations in the diameter/width of substrate W and in thethickness of the substrate W can be accommodated by deflections of theflexible further edge seal member 50. When liquid in the liquid supplysystem under the projection system PL passes over the edge of thesubstrate, the liquid cannot escape between the substrate W, flexiblefurther edge seal member 50 and edge seal member 117 because the edgesof those elements are tight against one another. Furthermore, becausethe primary surfaces of the substrate W and the edge seal member 117 andthe top surface of the flexible further edge seal member 50 aresubstantially co-planar, the liquid supply system operation is not upsetwhen it passes over the edge of the substrate W so that disturbanceforces are not generated in the liquid supply system.

As can be seen from FIG. 8, the flexible further edge seal member 50 isin contact with a surface of the substrate W opposite the primarysurface of the substrate W, at an edge portion. This contact has twofunctions. First, the fluid seal between the flexible further edge sealmember 50 and the substrate W may be improved. Second, the flexiblefurther edge seal member 50 applies a force on the substrate W in adirection away from the pimple table 20. When the substrate W is held onthe substrate table WT by, e.g. vacuum suction, the substrate can beheld securely on the substrate table. However, when the vacuum source isswitched off or disconnected, the force produced by the flexible furtheredge seal member 50 on the substrate W is effective to push thesubstrate W off the substrate table WT thereby aiding loading andunloading of substrates W.

The flexible further edge seal member 50 is made of a radiation andimmersion liquid resistant material such as PTFE.

Embodiment 5

FIG. 9 illustrates a fifth embodiment which is the same or similar asthe first embodiment except as described below.

This embodiment is described in relation to an edge seal member 117which is an integral part of the substrate table WT. However, thisembodiment is equally applicable to an edge seal member 17 which ismovable relative to the substrate table WT.

As can be seen from FIG. 9, the eighth embodiment includes a furtheredge seal member 100 for bridging the gap between the edge seal member117 and the substrate W. In this case the further edge seal member 100is a gap seal member which is positioned on the primary surfaces of thesubstrate W and the edge seal member 117 to span the gap between thesubstrate W and edge seal member 117. Thus, if the substrate W iscircular, the gap seal member 100 will also be circular (annular).

The gap seal member 100 may be held in place by the application of avacuum 105 to its underside (that is a vacuum source exposed through avacuum port on the primary surface of the edge seal member 117). Theliquid supply system can pass over the edge of the substrate W withoutthe loss of liquid because the gap between the substrate W and the edgeseal member 117 is covered over by the gap seal member 100. The gap sealmember 100 can be put in place and removed by the substrate handler sothat standard substrates and substrate handling can be used.Alternatively the gap seal member 100 can be kept at the projectionsystem PL and put in place and removed by appropriate mechanisms (e.g. asubstrate handling robot). The gap seal member 100 should be stiffenough to avoid deformation by the vacuum source. Advantageously the gapseal member 100 is less than 50, optionally 30 or 20 or even 10 μm thickto avoid contact with the liquid supply system, but should be made asthin as possible

The gap seal member 100 is advantageously provided with tapered edges110 in which the thickness of the gap seal member 100 decreases towardsthe edges. This gradual transition to the full thickness of the gap sealmember ensures that disturbance of the liquid supply system is reducedwhen it passes on top of the gap seal member 100.

The same way of sealing may be used for other objects such as sensors,for example transmission image sensors. In this case, as the object isnot required to move, the gap seal member 100 can be glued in place (ateither end) with a glue which does not dissolve in the immersion liquid.The glue can alternatively be positioned at the junction of the edgeseal member 117, the object and the gap seal member 100.

Furthermore, the gap seal member 100 can be positioned underneath theobject and an overhang of the edge seal member 117. The object may beshaped with an overhang also, if necessary.

The gap seal member 100, whether above or below the object, can have apassage provided through it, from one opening in a surface in contactwith the edge seal member 117 to another opening in a surface in contactwith the object. By positioning one opening in fluid communication withvacuum 105, the gap seal member 100 can then be kept tightly in place.

Embodiment 6

A sixth embodiment will be described with reference to FIG. 10. Thesolution shown in FIG. 10 bypasses some of the problems associated withimaging edge portions of the substrate W as well as allows atransmission image sensor (TIS) 220 (or other sensor or object) to beilluminated by the projection system PL under the same conditions as thesubstrate W.

The sixth embodiment uses the liquid supply system described withrespect to the first embodiment. However, rather than confining theimmersion liquid in the liquid supply system under the projection systemPL on its lower side with the substrate W, the liquid is confined by anintermediary plate 210 which is positioned between the liquid supplysystem and the substrate W. The spaces 222, 215 between the intermediaryplate 210 and the TIS 220 and the substrate W are also filled withliquid 111. This may either be done by two separate space liquid supplysystems via respective ports 230, 240 as illustrated or by the samespace liquid supply system via ports 230, 240. Thus the space 215between the substrate W and the intermediary plate 210 and the space 220between the transmission image sensor 220 and the intermediary plate 210are both filled with liquid and both the substrate W and thetransmission image sensor can be illuminated under the same conditions.Portions 200 provide a support surface or surfaces for the intermediaryplate 210 which may be held in place by vacuum sources.

The intermediary plate 210 is made of such a size that it covers all ofthe substrate W as well as the transmission image sensor 220. Therefore,no edges need to be traversed by the liquid supply system even when theedge of the substrate W is imaged or when the transmission image sensoris positioned under the projection system PL. The top surface of thetransmission image sensor 220 and the substrate W are substantiallyco-planar.

The intermediary plate 210 can be removable. It can, for example, be putin place and removed by a substrate handling robot or other appropriatemechanism.

All of the above described embodiments may be used to seal around theedge of the substrate W. Other objects on the substrate table WT mayalso need to be sealed in a similar way, such as sensors includingsensors and/or marks which are illuminated with the projection beamthrough the liquid such as a transmission image sensor, integrated lensinterferometer and scanner (wavefront sensor) and a spot sensor plate.Such objects may also include sensors and/or marks which are illuminatedwith non-projection radiation beams such as levelling and alignmentsensors and/or marks. The liquid supply system may supply liquid tocover all of the object in such a case. Any of the above embodiments maybe used for this purpose. In some instances, the object will not need tobe removed from the substrate table WT as, in contrast to the substrateW, the sensors do not need to be removed from the substrate table WT. Insuch a case the above embodiments may be modified as appropriate (e.g.the seals may not need to be moveable).

Embodiment 7

FIG. 11 shows a seventh embodiment which is the same as the firstembodiment except as described below.

In the seventh embodiment the object on the substrate table WT is asensor 220 such as a transmission image sensor (TIS). In order toprevent immersion liquid seeping underneath the sensor 220, a bead ofglue 700 which is undissolvable and unreactable with the immersion fluidis positioned between the edge seal member 117 and the sensor 220. Theglue is covered by immersion fluid in use.

Embodiment 8

An eighth embodiment is described with reference to FIGS. 12 and 13. Inthe eighth embodiment it is a sensor 220 which is being sealed to thesubstrate table WT. In both versions illustrated in FIGS. 12 and 13, avacuum 46 is provided adjacent the gap with an opening passage 47 and achamber 44 for taking away any immersion liquid which should find itsway through the gap between the edge seal member 117 and the edge of thesensor 220.

In the FIG. 12 version, the vacuum 46 is provided in the substrate tableWT under an overhang portion of the object 220. The passage 47 isprovided in an overhanging inwardly protruding portion of the substratetable WT. Optionally a bead of glue 700 is positioned at the inner mostedge of the protruding portion between the substrate table WT and theobject 220. If no bead of glue 700 is provided, a flow of gas fromunderneath the object 220 helps seal the gap between the sensor 220 andthe substrate table WT.

In the version of FIG. 13, the vacuum 46, compartment 44 and passage 47are provided in the object 220 itself under an inwardly protruding edgeseal member 117. Again there is the option of providing a bead of gluebetween the object 220 and the substrate table WT radially outwardly ofthe passage 47.

The shape of the edge seal member 117 and the top outer most edge of theobject 220 can be varied. For example, it may be advantageous to providean overhanging edge seal member 117 or indeed an outer edge of theobject 220 which is overhanging. Alternatively, an outer upper corner ofthe object 220 may be useful.

Example of High NA Detection Sensor

Substrate-level sensors according to one or more embodiments of theinvention may comprise a radiation-receiving element (1102, 1118) and aradiation-detecting element (1108, 1124) as shown in FIGS. 16-19.Exposure radiation is directed from the final element of the projectionsystem PL through an immersion liquid 11 at least partly filling a spacebetween the final element of the projection system PL and the substrateW. The detailed configuration of each of these elements depends on theproperties of the radiation to be detected. The sensor at substratelevel may comprise a photocell only, for use in cases where it isdesirable for the photocell to receive the radiation directly.Alternatively, the sensor at substrate level may comprise a luminescencelayer in combination with a photocell. In this arrangement, radiation ata first wavelength is absorbed by the luminescence layer and reradiateda short time later at a second (longer) wavelength. This arrangement isuseful, for example, where the photocell is designed to work moreefficiently at the second wavelength.

The radiation-receiving element (1112, 1118), which may be a layer witha pinhole, a grating or another diffractive element fulfilling a similarfunction, may be supported on top of a quartz sensor body 1120, i.e. onthe same side of the body as the projection system. Theradiation-detecting element (1108, 1124), in contrast, may be arrangedwithin the sensor body 1120, or within a concave region formed on theside of the sensor body 1120 facing away from the projection system.

At boundaries between media of different refractive indices a proportionof incident radiation will be reflected and potentially lost from thesensor. For optically smooth surfaces, the extent to which this occursdepends on the angle of incidence of the radiation and the difference inrefractive index of the media in question. For radiation incident at andabove a “critical angle” (conventionally measured from normal incidence)total internal reflection may occur, leading to serious loss of signalto later elements of the sensor. This may be a particular problem inhigh NA systems where radiation may have a higher average angle ofincidence. Accordingly, in an embodiment, arrangements are provided sothat gas is excluded from the region between the radiation-receiving(1102, 1118) and radiation-detecting (1108, 1124) elements in order toavoid interfaces between media of high refractive index and gas.

In addition to losses due to partial and total internal reflection,absorption may also seriously reduce the intensity of radiationintensity reaching the photocell, as may scattering from interfaces thatare not optically smooth.

FIG. 16 shows an integrated lens interferometer and scanner (ILIAS)sensor module. This module has a shearing grating structure 1102 as aradiation-receiving element, supported by a transmissive plate 1104,which may be made of glass or quartz. A quantum conversion layer 1106 ispositioned immediately above a camera chip 1108 (the radiation-detectingelement), which is in turn mounted on a substrate 1110. The substrate1110 is connected to the transmissive plate 1104 via spacers 1112 andbonding wires 1114 connect the radiation-detecting element to externalinstrumentation. A gas gap is located between the quantum conversionlayer 1106 and the transmissive plate 1104. In a setup such as thisdesigned for 157 nm radiation, for example, the gas gap within thesensor cannot easily be purged so that it will contain significantproportions of oxygen and water, which absorb radiation. Signal istherefore lost and the effect becomes worse for larger angles as thesehave a longer path length through gas. Thus, the dynamic rangerequirements for the sensor become more severe.

FIGS. 17 and 18 show improved ILIAS sensor modules according toembodiments of the present invention. In FIG. 17, the gas gap has beenremoved by changing the shape of the transmissive plate 1104 to fitdirectly to the camera 1108. This arrangement is made more difficult bythe need to provide access for the bonding wires 1114 and necessitatesan elongated form. From an engineering point of view, the alternativearrangement shown in FIG. 18 is easier to realize. Here, a filler sheet1116 of the same material as the transmissive plate 1104, or of similaroptical properties, is inserted between the transmissive plate 1104 andthe quantum conversion layer 1106. The removal of the gas gap reducestransmission losses and relaxes dynamic range requirements (or,alternatively speaking, improves the effective dynamic range). Botharrangements improve refractive index matching and reduce the extent ofspurious internal reflections at the interface with the transmissiveplate 1104.

FIG. 19 a shows a DUV transmission image sensor. FIG. 19 b shows amagnified view of the processing element for clarity. The pattern oftransmissive grooves 1118, constituting the radiation-receiving elementin this case, is realized by means of e-beam lithography and dry etchingtechniques in a thin metal layer deposited on a substrate by means ofsputtering. DUV radiation that is projected towards the grooves 1118 istransmitted by the transmissive plate 1104 (which may be quartz or fusedsilica) and hits the underlying luminescent material 1122, or“phosphor”. The luminescent material 1122 may comprise a slab ofcrystalline material that is doped with rare-earth ions, e.g.yttrium-aluminium-garnet doped with cerium (YAG:Ce). The main purpose ofthe luminescent material 1122 is to convert the DUV radiation into moreeasily detectable visible radiation, which is then detected by thephotodiode 1124. DUV radiation that has not been absorbed and convertedinto visible radiation by the phosphor 1122 may be filtered out beforeit reaches the photodiode 1124 (e.g. by a BG-39 or UG filter 1126).

In the above arrangement, gas may be present in the gaps betweencomponents mounted in the sensor housing 1125, yielding a number ofgas/material/gas interfaces that interrupt the propagation of radiation.By considering the path of DUV radiation and radiation arising fromluminescence, it is possible to identify regions where radiation islikely to be lost. The first region of interest is the rear-side 1128 ofthe transmissive plate 1104, reached by DUV radiation after it haspassed through the grooves 1118 and transmissive plate 1104. Here, thesurface has been formed by mechanical means, such as by drilling, and isinevitably rough on the scale of the wavelength of the radiation.Radiation may therefore be lost due to scattering, either back into thetransmissive plate 1104 or out past the luminescent material 1122.Secondly, after this interface, the DUV light encounters the opticallysmooth gas/YAG:Ce interface, where a substantial amount of reflectionmay occur due to the refractive index mismatch, particularly in systemsof high NA. Thirdly, the luminescent material 1122 emits radiation inrandom directions. Due to its relatively high refractive index, thecritical angle for total internal reflection at a YAG:Ce/air boundary isaround 33° (where, for example, there is air in the gap between theYAG:Ce and the filter 1126) from the normal, meaning that a largeproportion of radiation incident on the boundary is reflected out of thesystem and lost through the side walls of the luminescent material 1122.Finally, the part of the luminescence that is directed towards thephotodiode has to overcome the gas/quartz interface on the diode surfacewhere surface roughness may again account for loss of detected signal.

Each of the embodiments may be combined with one or more of the otherembodiments as appropriate. Further, each of the embodiments (and anyappropriate combination of embodiments) can be applied simply to theliquid supply system of FIG. 2 and FIGS. 14 and 15 without the edge sealmember 17, 117 as feasible and/or appropriate.

In an embodiment, there is provided a lithographic projection apparatuscomprising: an illuminator adapted to condition a beam of radiation; asupport structure configured to hold a patterning device, the patterningdevice configured to pattern the beam of radiation according to adesired pattern; a substrate table configured to hold a substrate; aprojection system adapted to project the patterned beam onto a targetportion of a substrate; a liquid supply system configured to at leastpartly fill a space between the projection system and an object on thesubstrate table, with a liquid; and a sensor capable of being positionedto be illuminated by a beam of radiation once it has passed through theliquid.

In an embodiment, the substrate table comprises a support surfaceconfigured to support an intermediary plate between the projectionsystem and the sensor and not in contact with the sensor. In anembodiment, the sensor comprises a transmission image sensor configuredto sense the beam and wherein the intermediary plate is positionablebetween the sensor and the projection system. In an embodiment, thesensor is on the substrate table. In an embodiment, the sensor comprisesan alignment sensor configured to align the substrate table relative tothe projection system. In an embodiment, measurement gratings of thealignment sensor have a pitch of less than 500 nm. In an embodiment, thealignment sensor is configured to be illuminated obliquely. In anembodiment, the sensor comprises a transmission image sensor. In anembodiment, the sensor comprises a focus sensor. In an embodiment, thesensor comprises a spot or dose sensor, an integrated lensinterferometer and scanner, an alignment mark, or any combination of theforegoing. In an embodiment, the substrate table comprises an edge sealmember configured to at least partly surround an edge of the sensor andto provide a primary surface facing the projection system substantiallyco-planar with a primary surface of the sensor. In an embodiment, thesensor is configured to be in contact with the liquid and the beam ofradiation is configured to come from the projection system or analignment system. In an embodiment, the beam of radiation is thepatterned beam. In an embodiment, an alignment system comprises thesensor and is configured receive an alignment beam of radiation from theprojection system to align the substrate. In an embodiment, thesubstrate table comprises a vacuum port configured to remove liquid froma space between the substrate table and the sensor. In an embodiment,the apparatus further comprises a bead of material in a space betweenthe substrate table and the sensor configured to prevent entry of theliquid.

In an embodiment, there is provided a device manufacturing methodcomprising: projecting a beam of radiation through a liquid onto asensor; and projecting the beam of radiation as patterned using aprojection system of a lithographic apparatus through the liquid onto atarget portion of a substrate.

In an embodiment, the liquid is supported on an intermediary platebetween the projection system and the sensor, the plate not being incontact with the sensor. In an embodiment, the sensor comprises atransmission image sensor configured to sense the beam and theintermediary plate is positionable between the sensor and the projectionsystem. In an embodiment, the sensor is on a substrate table holding thesubstrate. In an embodiment, the sensor comprises an alignment sensorconfigured to align a substrate table holding the substrate relative tothe projection system. In an embodiment, measurement gratings of thealignment sensor have a pitch of less than 500 nm. In an embodiment, thealignment sensor is configured to be illuminated obliquely. In anembodiment, the sensor comprises a transmission image sensor. In anembodiment, the sensor comprises a focus sensor. In an embodiment, thesensor comprises a spot or dose sensor, an integrated lensinterferometer and scanner, an alignment mark, or any combination of theforegoing. In an embodiment, a substrate table holding the substratecomprises an edge seal member configured to at least partly surround anedge of the sensor and to provide a primary surface facing theprojection system substantially co-planar with a primary surface of thesensor. In an embodiment, the method comprises projecting the beam ofradiation from the projection system or an alignment system through theliquid onto the sensor in contact with the liquid. In an embodiment, thebeam of radiation is the patterned beam. In an embodiment, an alignmentsystem comprises the sensor and is configured receive an alignment beamof radiation from the projection system to align the substrate. In anembodiment, a substrate table holding the substrate comprises a vacuumport configured to remove liquid from a space between the substratetable and the sensor. In an embodiment, the method further comprisesproviding a bead of material in a space between the substrate table andthe sensor configured to prevent entry of the liquid.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. In particular, the invention is also applicable toother types of liquid supply systems, especially localised liquid areasystems. If the seal member solution is used, it may be one in which aseal other than a gas seal is used. The description is not intended tolimit the invention.

1. A lithographic projection apparatus comprising: a substrate tableconfigured to hold a substrate; a projection system adapted to project apatterned beam onto a target portion of a substrate; a liquid supplysystem configured to at least partly fill a space between the projectionsystem and an object on the substrate table, with a liquid; and a sensorcapable of being positioned to be illuminated by a beam of radiationonce it has passed through the liquid.
 2. An apparatus according toclaim 1, wherein the substrate table comprises a support surfaceconfigured to support an intermediary plate between the projectionsystem and the sensor and not in contact with the sensor.
 3. Anapparatus according to claim 2, wherein the sensor comprises atransmission image sensor configured to sense the beam and wherein theintermediary plate is positionable between the sensor and the projectionsystem.
 4. An apparatus according to claim 1, wherein the sensor is onthe substrate table.
 5. An apparatus according to claim 1, wherein thesensor comprises an alignment sensor configured to align the substratetable relative to the projection system.
 6. An apparatus according toclaim 5, wherein measurement gratings of the alignment sensor have apitch of less than 500 nm.
 7. An apparatus according to claim 5, whereinthe alignment sensor is configured to be illuminated obliquely.
 8. Anapparatus according to claim 1, wherein the sensor comprises atransmission image sensor.
 9. An apparatus according to claim 1, whereinthe sensor comprises a focus sensor.
 10. An apparatus according to claim1, wherein the sensor comprises a spot or dose sensor, an integratedlens interferometer and scanner, an alignment mark, or any combinationof the foregoing.
 11. An apparatus according to claim 1, wherein thesubstrate table comprises an edge seal member configured to at leastpartly surround an edge of the sensor and to provide a primary surfacefacing the projection system substantially co-planar with a primarysurface of the sensor.
 12. An apparatus according to claim 1, whereinthe sensor is configured to be in contact with the liquid and the beamof radiation is configured to come from the projection system or analignment system.
 13. An apparatus according to claim 1, wherein thebeam of radiation is the patterned beam.
 14. An apparatus according toclaim 1, wherein an alignment system comprises the sensor and isconfigured receive an alignment beam of radiation from the projectionsystem to align the substrate.
 15. An apparatus according to claim 4,wherein the substrate table comprises a vacuum port configured to removeliquid from a space between the substrate table and the sensor.
 16. Anapparatus according to claim 4, further comprising a bead of material ina space between the substrate table and the sensor configured to prevententry of the liquid.
 17. A device manufacturing method comprising:projecting a patterned beam of radiation through a liquid onto a sensor;and projecting a beam of radiation using a projection system of alithographic apparatus through the liquid onto a radiation-sensitivetarget portion of a substrate.
 18. A method according to claim 17,wherein the liquid is supported on an intermediary plate between theprojection system and the sensor, the plate not being in contact withthe sensor.
 19. A method according to claim 18, wherein the sensorcomprises a transmission image sensor configured to sense the patternedbeam and the intermediary plate is positionable between the sensor andthe projection system.
 20. A method according to claim 17, comprisingprojecting the patterned beam of radiation from the projection system oran alignment system through the liquid onto the sensor in contact withthe liquid.